1. Field of the Invention
This invention relates in general to the field of seismic exploration and in particular to seismic exploration which is utilized to investigate the strata surrounding a borehole. Still more particularly, the present invention relates to seismic exploration methods which utilize a seismic source which is located within a borehole.
2. Description of the Prior Art
Seismic exploration methods are well known in the prior art. Seismic exploration is normally utilized to obtain broad general information about subsurface strata. These techniques are generally implemented by utilizing an explosive or vibratory source which is disposed at multiple locations on the surface of the earth and a plurality of geophone sensors which are laid out at regular intervals near each source or "shot point".
Well logging techniques are also well known in the art and are generally utilized to obtain additional information about the strata which surround the borehole. While highly detailed information concerning porosity, permeability and other factors may be obtained by utilizing these techniques, the information is generally limited to the vicinity of the borehole.
More recently, it has been discovered that a combination of seismic exploration techniques with selected borehole measurements may be utilized to obtain more detailed information over a broader area of investigation. This technique is often referred to as Vertical Seismic Profiling (VSP), and involves the placement of geophone sensors in the borehole and the utilization of one or more shot points on the surface near the borehole. This technique can provide considerable additional information to help delineate subsurface formations and reservoirs. However, the information provided is frequently sparse and can be quite expensive to obtain.
A single source VSP setup can often take three to six hours to rig and more complex multi-offset-source setups can take several days. Drilling rig operators do not generally wish to incur the costs involved with these techniques for the amount of information received. Additionally, the high cost associated with the surface sources and recording systems can add significantly to the cost associated with this technique. It should therefore be obvious that an improved technique must be discovered that provides similar information without the high costs involved with VSP.
One approach to solving the aforementioned problem is the so-called "Inverse VSP" technique, in which the geophones are disposed on the surface of the earth and a seismic source is utilized within the borehole. Among the problems associated with this technique is the provision of a seismic source which is sufficiently repeatable and powerful for a useful acoustic wavefield to be detected at the surface and sufficiently robust to permit easy recording of the resultant seismic waves and which does not damage the borehole or the downhole equipment. If this problem can be solved, it will be possible to conduct three-dimensional surveys quite easily, due to the possibility of multiple offset and azimuth measurements which can be accomplished with this technique.